Feature/17 implement service accounts

[maikelpoot]

This PR will allow the listing and creation of Services accounts:

List All:

res, _ := client.ServiceAccounts.List()
for _, sa := range res {
	fmt.Println("Service account: ", sa.Name, sa.System.Name, sa.System.Type)
}

Create new

newSA := model.NewServiceAccount()
newSA.Name = "A Service Account"
newSA.Username = "service-account"

newSA.System = provSys
newSA.TechnicalAdministrator = group

tmp, _ := client.ServiceAccounts.Create(newSA)

Update existing

existingSa, _ := client.ServiceAccounts.GetByUUID(uuid.MustParse("00000-00-000"))
existingSa.Name = "Example"
existingSa.Username = "example"

updatedSA, _ := client.ServiceAccounts.Update(existingSa)

Delete

// By Object
existingSa, _ := client.ServiceAccounts.GetByUUID(uuid.MustParse("00000-00-000"))
_ := client.ServiceAccounts.Delete(existingSa)
// By UUID
_ := client.ServiceAccounts.DeleteByUUID(uuid.MustParse("00000-00-000"))
// By ID
_ := client.ServiceAccounts.DeleteByID(1337)